The present invention refers to a current generator able to provide in alternative to a first terminal a first current and a second current in response to a control signal. Particularly it refers to a current generator usable for the adaptive biasing of modulators of the sigma-delta type. It refers besides to a modulator of the sigma-delta type having an adaptive current generator.
To elaborate an analogical signal by means of digital circuits it is necessary to use a converter to convert the analogical signal into a digital signal. The modern technology requires fast and accurate conversion circuits. Lately the modulators of the sigma-delta type, which constitute a large part of the converters, have been much used thanks to their excellent performances.
The sigma-delta modulators comprise, as basic elements, some operational amplifiers which are normally of the AB class. In the case in which the pass band of the operational amplifiers is very wide the power consumption gets up fearfully, adaptive bias circuits able to reduce such consumption have therefore been studied.
The operational amplifiers of a sigma-delta modulator have an operation divided in two steps. A first step of sampling signal in which the requirements in terms of band are not elevated and it is therefore possible to have a reduced current consumption. Besides in this step in which the greatest worry is that the operational amplifier is stable, by reducing the current consumption it is possible to get also an increase of the phase margin. A second step in which the signal is integrated, in this case the maximum width of the available band is necessary and consequently it is not possible to limit the current consumption.
There are therefore known circuits that provide a feeding to these operational amplifiers according to the two above steps and that is an elevated current during the integration step and a reduced current during the sampling step. Such circuits are based mainly on the fact of having two current generators, one directly connected to the operational amplifier and the other connectable on order. Such circuits are however usable only for relatively low working frequency. As the working frequencies increase more elevated current and very fast response time are necessary, and the circuits based on the preceding type circuits are not able to get such performances.
In view of the state of the art described, it is an object of the present invention to provide a current generator usable for the adaptive biasing in modulators of the sigma-delta type suitable for elevated frequencies and without the drawbacks of the known art.
According to the present invention, such object and others are achieved by means of a current generator able to provide in alternative to a first terminal a first current and a second current in response to a control signal, characterized by comprising: a first current generator able to provide said first current; a second current generator able to provide said second current; commutation means able to connect in alternative to said first terminal said first current and said second current in response to said control signal.
According to the present invention, such objects are also reached by means of a sigma-delta modulator comprising at least an operational amplifier able to work alternatively as an integrator and as a sampler and having a first bias current and a second bias current, characterized by comprising a current generator, according to claim 1, able to provide in alternative to a supply terminal of said operational amplifier said first current and said second current in response to a control signal.